Optical disks, magneto-optical disks, etc. are used in memory devices for computers, and as packaged media for music and video information. In optical recording and reproducing devices for this type of disk, a light beam projected by a light source is converged by an objective lens, and projected onto a recording layer of the disk, where recording and reproducing are performed (this type of optical recording and reproducing device will hereinafter be referred to as "Conventional Example 1").
In recent years, there is a demand for increased recording density in such optical recording and reproducing devices. One method of achieving increased recording density is to reduce the diameter of the light spot projected onto the recording layer of the optical recording medium.
The reason behind this is that, when reproducing an information signal from an optical recording medium recording small recording marks at high density, a smaller light spot diameter gives rise to less contamination of the signal by signals from marks adjacent to a mark to be reproduced (so-called "crosstalk"), and the small recording marks can thus be reproduced accurately. Further, in recording an information signal in the recording medium, a smaller light spot diameter enables small marks to be recorded accurately, without influencing adjacent marks.
However, the spot diameter of a light beam is proportional to .lambda./NA, where .lambda. is the wavelength of the light and NA is the numerical aperture. Therefore, in order to reduce the diameter of a light beam spot, it suffices to increase the numerical aperture of the objective lens which converges the light beam onto the surface of the recording medium. However, due to the difficulty of manufacturing objective lenses, there is a limit to how much the numerical aperture can be increased (practically, to around 0.6).
One proposed solution to this difficulty is to decrease the spot diameter by using a composite of lenses for the objective lens (hereinafter referred to as an "objective lens composite"). The following will explain such an objective lens composite in concrete terms with reference to FIG. 5.
As shown in FIG. 5, this type of objective lens composite includes an objective lens 200 having a numerical aperture of NA, and a hemispherical lens 201 having a refractive index of N. A parallel light beam P1 is projected onto the objective lens 200, which reduces the beam diameter thereof and projects a converged light beam P2. The hemispherical lens 201 has a hemispherical light-incident surface facing the objective lens 200, and is positioned such that rays of the light beam P2 strike the foregoing light-incident surface perpendicularly. Further, the opposite surface of the hemispherical lens 201 from the light-incident surface is flat.
In an objective lens composite with the foregoing structure, since the rays of the light beam P2 exiting the objective lens 200 strike the light-incident surface perpendicularly, there is little reflection or diffraction of these rays as they enter the hemispherical lens 201. Accordingly, as the light beam P2 enters the hemispherical lens 201, it maintains the angle with which it was converged by the objective lens 200 of numerical aperture NA. Here, since the hemispherical lens 201 has a refractive index of N, the wavelength of the light after entering the hemispherical lens 201 is 1/N.
Then, light exiting the flat surface of the hemispherical lens 201 is further converged due to a difference in refractive indices of the hemispherical lens 201 and air, exiting as a light beam P3 corresponding to a numerical aperture of N.times.NA (wavelength returns to .lambda.).
In this way, with an objective lens composite like that shown in FIG. 5, a light beam effectively having a large numerical aperture can be easily produced. Various optical disk devices using this type of objective lens composite have been proposed.
An optical disk device using such an objective lens composite, shown in FIG. 6, is disclosed in Japanese Unexamined Patent Publication Nos. 8-221772/1996 (Tokukaihei 8-221772) and 8-221790 (Tokukaihei 8-221790) (the optical disk device disclosed in these publications will be referred to hereinafter as "Conventional Example 2").
In the foregoing conventional optical disk device, light projected from an objective lens composite 210, made up of an objective lens 200 and a hemispherical lens 201, reaches an optical disk 211 across a gap 212 of at least several .mu.m, and is projected onto a recording layer 213, in which information is recorded. Here, light projected from the objective lens composite 210 crosses the gap 212 and reaches the optical disk 211 as a light beam corresponding to a numerical aperture of N.times.NA, as discussed above.
With the foregoing conventional optical disk device, the light beam projected onto the optical disk 211 has a numerical aperture N times greater (N=refractive index of the hemispherical lens 201), i.e., a beam spot with a diameter of 1/N times that when the objective lens 200 is used alone.
Another optical disk device using an objective lens composite, shown in FIG. 7, is disclosed in Nikkei Electronics, Jun. 16, 1997, pp. 99-108 (hereinafter referred to as "Conventional Example 3").
In the foregoing conventional optical disk device, an objective lens composite 210, made up of an objective lens 200 (numerical aperture=NA) and a hemispherical lens 201 (refractive index=N), is positioned in close proximity (around .lambda./4) to a recording layer 213 of an optical disk 211.
If the objective lens composite 210 and the recording layer 213 are positioned in close proximity with one another, near field effect causes light attempting to exit the flat surface of the hemispherical lens 201 to seep through the flat surface and reach the recording layer retaining the same properties it had inside the hemispherical lens 201.
As mentioned above, while inside the hemispherical lens 201, the light beam has a numerical aperture of NA and a wavelength of 1/N of its initial wavelength. Accordingly, the light beam which reaches the recording layer 213 has a wavelength of 1/N times that of normal projected light. Therefore, the light beam projected onto the recording layer 213 has a beam spot of 1/N the diameter of normal projected light.
In this way, Conventional Example 3 makes use of near field effect to guide the light beam, whose wavelength has been reduced by the hemispherical lens 201, to the recording layer 213 with unchanged properties, thus reducing the size of the beam spot. An example of use of such an objective lens composite in a lithography system is disclosed in U.S. Pat. No. 5,121,256.
As discussed above, with Conventional Examples 2 and 3, the size of the beam spot projected onto the recording layer can theoretically be reduced, thus realizing high density of information recording in the optical disk.
However, in the case of Conventional Example 2 shown in FIG. 6, if the numerical aperture of the light beam projected from the objective lens composite 210 is too large, rays near the perimeter of the light beam exiting the hemispherical lens 201 have a large angle of incidence at the flat surface of the hemispherical lens 201, and are totally reflected therefrom. Thus there is a limit to how much the numerical aperture can be increased.
For example, reflectance at the interface between the hemispherical lens 201 (refractive index=1.5) and air (refractive index=1.0) begins to increase at an angle of incidence of around 33.degree., and is totally reflected at an angle of incidence of 41.8.degree..
An increase in reflectance means that less light reaches the optical disk 211, and with total reflection, no light is projected onto the optical disk 211. For this reason, with Conventional Example 2, there is a limit to how much the numerical aperture can be increased to improve recording density. Practically, the numerical aperture cannot be increased to more than about 0.85.
Further, with Conventional Example 3 shown in FIG. 7, in order for the light, whose wavelength is reduced to 1/N of its wavelength in air, to be guided to the recording layer while maintaining that wavelength, it is necessary to position the recording layer 213 and the hemispherical lens 201 so that a distance therebetween is around 1/4 of the initial wavelength of the light, and in this case, the optical disk 211 as a recording medium cannot be provided with an effective protective film. Therefore, operations are greatly influenced by dust, and even dust on the order of the wavelength of the light not only influences operations, but may also damage the optical disk. Further, if the optical disk 211 as a recording medium is made air-tight to avoid dust, one characteristic advantage of optical disks, namely, substitution of different disks, is lost.
A further problem with the optical disk device in Conventional Example 3 is that, since the efficiency of optical coupling between the hemispherical lens 201 and the recording layer 213 is reduced to approximately 50%, a sufficient quantity of information light cannot be obtained. The reason for this, the present inventors believe, is that in Comparative Example 3, the light beam whose wavelength has been reduced (to 1/N of its initial wavelength) in the hemispherical lens 201 exits into air through the flat surface.
At this time, the light beam diameter (the diameter of that part of the light beam where light quantity is 1/e.sup.2 of peak intensity) is smaller than the wavelength in air. Accordingly, since the light beam has a diameter which cannot usually exist in air, if the interval between the hemispherical lens 201 and the recording layer is too large, the light apparently undergoes some sort of loss after exiting into the air.